Spacecraft perform various maneuvers after they are launched into space and once they are on-station in an intended orbit. After the spacecraft is on-station in a selected orbit, various forces (e.g., solar and/or other environmental disturbance torques, such as magnetic torques) may act on the spacecraft and cause the spacecraft to drift away from its selected orbit into another, incorrect orbit. Thus, periodic (e.g., daily, weekly, or monthly) orbital maneuvers are often required to return the spacecraft to the correct orbit. These types of maneuvers are known as station-keeping maneuvers.
During the performance of each type of maneuver, the precise control of the spacecraft's attitude is essential to orient the spacecraft's payload, such as communication or imaging hardware, to a preselected planetary location and/or to correctly orient the spacecraft's thrust vector. Thus, spacecraft are typically equipped with closed-loop control systems which enable the attitude of the spacecraft to be controlled within pre-established deadband limits. Such control systems often employ spacecraft thrusters for selectively producing torques on the spacecraft for correcting the spacecraft attitude.
The following commonly assigned U.S. patents are illustrative of various approaches to providing spacecraft attitude control: U.S. Pat. No. 5,459,669, Control System And Method For Spacecraft Attitude Control, to Adsit et al.; U.S. Pat. No. 5,400,252, Spacecraft East/West Orbit Control During A North Or South Stationkeeping Maneuver, to Kazimi et al.; U.S. Pat. No. 5,349,532, Spacecraft Attitude Control And Momentum Unloading Using Gimballed And Throttled Thrusters, to Tilley et al.; and U.S. Pat. No. 5,222,023, Compensated Transition For Spacecraft Attitude Control, to Liu et al.
Reference can also be had to U.S. Pat. No. 5,184,790, Two-Axis Attitude Correction For Orbit Inclination, to Fowell; U.S. Pat. No. 4,931,942, Transition Control System For Spacecraft Attitude Control, to Garg et al.; U.S. Pat. No. 4,848,706, Spacecraft Attitude Control Using Coupled Thrusters, Garg et al.; U.S. Pat. No. 4,767,084, Autonomous Stationkeeping For Three-Axis Stabilized Spacecraft, to Chan et al.; U.S. Pat. No. 4,599,697, Digital PWPF Three Axis Spacecraft Attitude Control, to Chan et al.; U.S. Pat. No. 4,521,855, Electronic On-Orbit Roll/Yaw Satellite Control, to Lehner et al.; U.S. Pat. No. 4,489,383, Closed-Loop Magnetic Roll/Yaw Control System For High Inclination Orbit Satellites, to Schmidt, Jr.; and U.S. Pat. No. 4,084,772, Roll/Yaw Body Steering For Momentum Biased Spacecraft, to Muhlfelder.
Also of interest is U.S. Pat. No. 4,759,517, Station-Keeping Using Solar Sailing, to Clark; and U.S. Pat. No. 4,684,084, Spacecraft Structure with Symmetrical Mass Center and Asymmetrical Deployable Appendages, to Fuldner et al.
Reference is also made to a publication entitled "Attitude Stabilization of Flexible Spacecraft During Stationkeeping Maneuvers", Bong Wie et al., J. Guidance, Vol. 7, No. 4, pgs. 430-436, July-August 1984.
A typical geosynchronous satellite is designed to minimize solar torque imbalance. This is typically accomplished with symmetric solar array design, with solar arrays being located on the north and south side of the spacecraft, or in a configuration with the solar array located on the south side, balanced by a solar sail on the north side. These appendages extend from a spacecraft bus. Residual solar and environmental disturbance torques are stored in momentum wheels that are then unloaded periodically using, by example, the spacecraft's thrusters, magnetic torquers, trim tabs, or solar panel angle adjustments.
It can be appreciated that a technique to control inertial roll and yaw solar torques is an important aspect of spacecraft operation, particularly for spacecraft having payloads, such as imaging or communication payloads, that require a high degree of short term attitude stability.